Amazon(AMZN) didn't immediately respond to requests for comment on when and why the threshold was lowered.

Shoppers with Amazon Prime memberships, which cost $10.99 a month or $99 a year, get unlimited free two-day shipping on eligible items with no minimum along with other benefits like streaming TV, movies and music.

With more consumers shopping from their computers, online retailers have been battling it out to gain and retain customers.